title: Roles of RIG-I N-terminal tandem CARD and splice variant in TRIM25-mediated antiviral signal transduction.
abstract: The caspase recruitment domain (CARD) of intracellular adaptors and sensors plays a critical role in the assembly of signaling complexes involved in innate host defense against pathogens and in the regulation of inflammatory responses. The cytosolic receptor retinoic acid-inducible gene-I (RIG-I) recognizes viral RNA in a 5'-triphosphate-dependent manner and initiates an antiviral signaling cascade. Upon viral infection, the N-terminal CARDs of RIG-I undergo the K(63)-linked ubiquitination induced by tripartite motif protein 25 (TRIM25), critical for the interaction of RIG-I with its downstream signaling partner MAVS/VISA/IPS-1/Cardif. Here, we demonstrate the distinct roles of RIG-I first and second CARD in TRIM25-mediated RIG-I ubiquitination: TRIM25 binds the RIG-I first CARD and subsequently ubiquitinates its second CARD. The T(55)I mutation in RIG-I first CARD abolishes TRIM25 interaction, whereas the K(172)R mutation in the second CARD eliminates polyubiquitin attachment. The necessity of the intact tandem CARD for RIG-I function is further evidenced by a RIG-I splice variant (SV) whose expression is robustly up-regulated upon viral infection. The RIG-I SV carries a short deletion (amino acids 36-80) within the first CARD and thereby loses TRIM25 binding, CARD ubiquitination, and downstream signaling ability. Furthermore, because of its robust inhibition of virus-induced RIG-I multimerization and RIG-I-MAVS signaling complex formation, this SV effectively suppresses the RIG-I-mediated IFN-beta production. This study not only elucidates the vital role of the intact tandem CARD for TRIM25-mediated RIG-I activation but also identifies the RIG-I SV as an off-switch regulator of its own signaling pathway.
authors.affiliation: Department of Molecular Microbiology and Immunology, Keck School of Medicine, University of Southern California, Harlyne J. Norris Cancer Research Tower, 1450 Biggy Street, Los Angeles, CA 90033, USA.
authors.forename: Michaela U
authors.initials: MU
authors.lastname: Gack
authors.forename: Axel
authors.initials: A
authors.lastname: Kirchhofer
authors.forename: Young C
authors.initials: YC
authors.lastname: Shin
authors.forename: Kyung-Soo
authors.initials: KS
authors.lastname: Inn
authors.forename: Chengyu
authors.initials: C
authors.lastname: Liang
authors.forename: Sheng
authors.initials: S
authors.lastname: Cui
authors.forename: Sua
authors.initials: S
authors.lastname: Myong
authors.forename: Taekjip
authors.initials: T
authors.lastname: Ha
authors.forename: Karl-Peter
authors.initials: KP
authors.lastname: Hopfner
authors.forename: Jae U
authors.initials: JU
authors.lastname: Jung
mesh_terms: D053468:CARD Signaling Adaptor Proteins / Q000276:immunology; D045744:Cell Line, Tumor; D000071457:DEAD Box Protein 58; D053487:DEAD-box RNA Helicases / Q000235:genetics / Q000276:immunology* / Q000378:metabolism; D006801:Humans; D007113:Immunity, Innate; D009154:Mutation; D020033:Protein Isoforms; D011971:Receptors, Immunologic; D015398:Signal Transduction / Q000276:immunology; D014157:Transcription Factors / Q000378:metabolism*; D000070584:Tripartite Motif Proteins; D044767:Ubiquitin-Protein Ligases / Q000378:metabolism*; D054875:Ubiquitination; D015854:Up-Regulation / Q000276:immunology
chemical_list: D053468:CARD Signaling Adaptor Proteins; D020033:Protein Isoforms; D011971:Receptors, Immunologic; D014157:Transcription Factors; D000070584:Tripartite Motif Proteins; C084523:TRIM25 protein, human; D044767:Ubiquitin-Protein Ligases; C452820:RIGI protein, human; D000071457:DEAD Box Protein 58; D053487:DEAD-box RNA Helicases